310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
tinually therefrom had collected in the voleanic chimney and blew 
out the magma; the other that the volatile materials were acquired 
by the igneous magma where in contact with water-bearing rocks 
in the upper strata of the earth’s crust. 
Were the former the case one would expect that the first products 
of an eruption should be less gas filled or gas bearing than the latter, 
but this is not the case. My observations, which demonstrate the 
fundamental facts upon which eruptive action of a volcano depends, 
shows unmistakably that the first materials yielded in a normal 
eruption after a long period of “ repose ” of a volcano are the richest 
in volatile elements, and that as the eruption proceeds the amount of 
gases in the issuing magma steadily diminishes, as shown by the 
diminished vesicularity and increased crystalline individualization of 
the essential ejecta. 
The class which interests us in the present question is the group of 
the essential ejecta. I found that when one examines the stratified 
.deposits of the ejecta thrown out during an explosive eruption—that 
is, an eruption of great violence taking place after a long period of 
repose of an old volcano, or the initial outburst of a new one—the 
materials, as they fell from the air, show a definite arrangement, 
and vary in character in correspondence with different phases of the 
eruption. 
To illustrate what these characters are I propose to choose a 
classical example in that of the great outburst of Vesuvius that over- 
whelmed Pompeii, Herculaneum, Stabia, Oplontis, and other towns 
around the foot of Somma Vesuvius. If we examine the deposit of 
materials that fell during the eruption of A. D. 79 in the streets of 
Pompeu, or preferably outside the town, as the falling houses have 
disturbed the regularity of the stratification within the walls, we 
find it made up of several beds. Immediately reposing on the old 
land surface is a stratum of very white light pumice. I use the word 
“white ” in comparison with that above it. If we collect a quantity 
of this we shall see that its bulk is very great for its weight. To the 
naked eye it is composed for the most part of a glassy vesicular base, 
with here and there scattered crystals of felspars, hornblende, 
pyroxene, and biotite, besides occasionally extraneous minerals 
7JIn my paper ‘“ On the Fragmentary Hjecta of Voleanoes” (Proc. Geol. Assoc., 
vol. 9, pp. 421-482 and 8 figs.) I divided such ejecta into three classes. Hssential 
ejecta are those materials that issue in a fluid state, and consist either of the 
volatile constituents or the magma in which these were contained, that produced 
the particular emission in question. Accessory ejecta consist of the older 
voleaniec materials of the same vent torn away, expelled, and mixed with the 
essential ejecta of an eruption. Accidental ejecta consist of either volcanic 
materials from other centers, or sedimentary or other rocks of the subvolcanic 
platform, also torn out, expelled, and mixed with the two before-mentioned 
ejecta. 
